Today's best guess is that there are about 100 thousand million million million planets in the visible universe. You don't need Steven Spielberg's imagination to suspect that there could be life out there somewhere.

Some of it could be nearby. For years, space agencies in Europe and the US have pummelled Mars with robotic probes, looking for clues that water once gushed and pooled on the red planet. As we know, life needs water. When you get past all the mission-justifying techno-babble, the most intriguing thing about Mars is the possibility that it once spawned living things - and may still have them.

But even so, nobody expects the Martians to be the sort of life you see at your local zoo. Red planet residents will be microscopic, enduring a limited lifestyle in moist rock hundreds of feet below the rusty landscape. Perhaps you find that unimpressive. However, discovery of microbes, dead or alive, on this nearby world would strongly suggest that life is not a miracle - just a natural chemical process that could infect countless planets and moons. If so, then couldn't there also be many worlds with intelligent life?

This possibility is now being investigated by small groups of scientists in an experiment known as Seti - the Search for Extraterrestrial Intelligence. Since distance currently prohibits finding them the way we might uncover the microscopic Martians, and evidence for their visiting us is anecdotal and ambiguous, Seti attempts to find the aliens at home, by eavesdropping on signals - specifically light or radio - that may either be accidentally leaking off their world (similar to the way television signals inevitably leak off ours), or are being deliberately sent our way. Our radio transmitters and lasers are detectable tens of light years away. If we could transmit to them, surely it is possible that signals from older, more advanced civilisations than ours could be washing over our planet now.

As yet Seti has failed to turn up a single convincing signal, despite decades of listening. But the hunt goes on. Unlike the impression you might have from movies or TV, the Seti scientists don't sit around with headphones, hopefully waiting for a squeal from the stars. Today's Seti searches use high-tech, digital receivers to monitor upwards of 100 million channels simultaneously.

And Seti is now building its own telescopes, rather than borrowing occasional bits of time on someone else's instruments. For example, at Harvard University, there's a specialised optical telescope scanning the sky every clear night, looking for faint, flashing laser pulses, while Seti and the University of California at Berkeley are constructing the Allen Telescope Array - a large collection of relatively small (6 metre) antennas for radio eavesdropping. This instrument will fundamentally change the Seti game. Over the course of the next 25 years, it will listen in the directions of a million nearby stars - a thousand times as many as we've carefully examined so far.

Consequently, it might happen that one morning in the not-too-distant future, you'll read about a discovery. No danger in that - they'll be extraordinarily far away. But at least you'll know something that none of our forebears ever did: that humans aren't the only cosmic inhabitants clever enough to examine nature, and put it to use.